(1) Field of the Invention
The present invention is directed toward methods for processing effluent from the chloromethylation of vinyl aromatic polymers, including the use of distillation to remove and/or recover volatile organics.
(2) Description of the Related Art
Chloromethylation of vinyl aromatic polymers is a common industrial process involving an addition of a chloromethyl (—CH2Cl) functional group to an aromatic ring of a vinyl aromatic polymer. The reaction is generally conducted by combining a vinyl aromatic polymer (e.g. a styrene-divinylbenzene copolymer) with a chloromethylation reagent (e.g. chloromethyl methyl ether) in the presence of Lewis acid catalyst (e.g. ferric chloride). The reaction product is a chloromethylated vinyl aromatic polymer which is useful in a variety of commercial applications including use as an intermediate in the production of anion exchange resins.
The reaction product mixture (“effluent”) from such chloromethylation reactions typically comprises catalyst, hydrochloric acid, and non-volatile organics. The effluent also includes valuable and/or environmentally sensitive constituents including volatile organics such as chloromethyl methyl ether (CMME), methylal, formaldehyde and methanol. In many processes, a portion of the volatile organics are recovered from the effluent. For example, U.S. Pat. No. 4,568,700 describes a method including filtering the chloromethylated polymer product from the effluent followed by treatment of the effluent by addition of hydrochloric acid to deactivate the catalyst and the subsequent addition of water. Alcohol is recovered from the resulting effluent via distillation or dialysis. Similarly, an abstract of Romanian Patent No. 79140 describes the addition of hydrochloric acid and formaldehyde to the effluent followed by distillation to recover a fraction having a boiling point (bp) of from 35 to 105° C. By way of another example, Japanese Patent Publication No. 61204/1987 (as described in U.S. Pat. No. 5,600,022, comparative example 1) describes the addition of hydrochloric acid to effluent followed by distillation at 98° C. under standard atmospheric pressure.
U.S. Pat. No. 4,636,554 describes the addition of 20 to 35% hydrochloric acid to chloromethylation effluent to suppress hydrolysis of CMME along with deactivating the catalyst. The effluent is then distilled to recover CMME along with other volatile organics such as methanol, methylal and formaldehyde. The distillation is usually conducted under conditions such that the dispersing medium, i.e. water or high boiling organic solvent is not distilled. This reference also describes an alternative addition of hydrogen chloride gas rather than hydrochloric acid. As hydrogen chloride gas does not deactivate the catalyst, a basic substance is added. The remaining effluent (distillation residue) includes significant amounts of volatile organics including both CMME and methanol.
As yet another example, U.S. Pat. No. 4,900,796 describes a chloromethylation process including the in-situ generation of CMME by the addition of methanol, formaldehyde and hydrochloric acid to the effluent. CMME is distilled from the resulting effluent mixture at atmospheric or mild vacuum pressure up to 70° C. followed by reduced vacuum (i.e. 300 mm to 600 mm). Reclaimed CMME is recycled for use in subsequent chloromethylation reactions.
U.S. Pat. No. 6,756,462 describes the use, recycle and reuse of sulfuric acid in an in-situ chloromethylation reaction of vinyl aromatic polymers. The reference describes the effluent from the reaction including a mixture of sulfuric acid, unreacted CMME, methanol, water, ferric chloride, methylal and other reaction byproducts including iron sulfate complexes, oligomers of formaldehyde and polystyrene derivatives.
British Patent No. 1,162,078 describes an earlier process including the pre-treatment of effluent with soda to form carbonate precipitates, followed by distillation to recover organic substances. The distillation was apparently difficult due to the formation of formaldehyde condensation products (i.e. fouling) of the plates of the distillation column resulting in lower yields. The reference also describes an “improved” process for recovering volatile organics from the effluent including the step of adding methanol prior to fractional distillation for the purpose of increasing recovery of methylal and methanol. The still bottoms remaining after the distillation are heated to carbonize organic impurities, the solids are then separated from the effluent and the remaining solution is evaporated to recover chlorides. Contrary to its teaching, the disclosed conditions for carbonizing organic impurities (i.e. heat the material to temperatures from 100 to 150° C.) more likely crosslinks and precipitates the chloromethylated extracted polymer residue. That is, the temperatures described are believed to be insufficient for carbonization and more likely result in the formation of an insoluble organic tar that is difficult to filter and readily fouls distillation equipment. Example 1 describes a recovery of 92 to 95% of methylal and methanol; however, it appears the recovery values were based upon the assumption that all CMME is converted to methylal and the remaining methylal and methanol being recoverable. It is more likely that a portion of CMME reacts with methanol according to Equation 1.ClCH2OCH3+CH3OHCH3OCH2OCH3+HCl  (1)Thus, it appears that the loss in weight due to formation of hydrogen chloride was ignored in the recovery calculation. When hydrogen chloride is considered, recovery values drop significantly from those reported.
U.S. Pat. No. 5,600,022 describes an undesired result of earlier processing methodologies that include the addition of hydrochloric acid followed by distillation. More specific, this reference provides that, CMME in the presence of concentrated hydrochloric acid under heating conditions for distillation results in undesired side reactions including the conversion of CMME (and methanol and formaldehyde) to methyl formate and methyl chloride. The reference goes on to describe an alternative method including the addition of hydrochloric acid and an extraction solvent followed by hydrogen chloride gas. The resulting organic and aqueous layers are then separated. By utilizing solvent extraction rather than distillation, the formation of methyl formate and methyl chloride are reduced. The CMME is finally separated from the organic layer via distillation, membrane separation, solvent extraction or chromatographic separation.
Each of the aforementioned references are incorporated herein in their entirety.
Despite the use of recycle loops and/or various separation techniques at least a portion of the effluent remaining from the chloromethylation must ultimately be disposed of. This is most typically accomplished via conventional waste water treatment. Environmental regulations are imposing increasingly demanding recovery limits for volatile organics in effluent. For example, title 40 of the US Code of Federal Regulations (40 CFR 63, subpart FFFF) establishes new standards for methanol emissions effective in 2008. While traditional distillation techniques remove a majority of volatile organics from chloromethylation effluent, the use of higher distillation temperatures (e.g. near or in excess of about 80° C. at standard atmospheric pressure) sufficient to remove greater quantities of volatile organics often leads to fouling of the distillation equipment with non-volatile organic materials (i.e. organic tars) and catalyst. Removal of such foulants from distillation equipment is difficult and imposes a practical limit on distillation techniques. Thus, new methodologies are sought which are capable of removing high quantities of volatile organics from effluent without the deleterious fouling of distillation equipment.